The present invention relates to a digital control arrangement for adjusting the bandwidth of an optical receiver and, more particularly, to an arrangement for modifying a parametric value (e.g., voltage, current, capacitance, etc.) applied to a preamplifier so as to tune the output bandwidth of the receiver.
In certain optical communication systems, it has become important to control the bandwidth of the optical receiver. It is often desirable to maintain a bandwidth only as high as necessary to recover the information contained in the signal, thereby filtering out noise at higher frequencies. In particular, many optically amplified communication systems, especially wavelength division multiplexed (WDM) systems, have extremely tight tolerances on the receiver bandwidth in order to use the optical receiver to filter out amplified spontaneous emission noise.
Conventional optical receivers may exhibit a bandwidth that is too large for some WDM applications. One solution to this problem is to modify the fabrication process for optical receivers (usually GaAs integrated circuit devices) so as to produce receivers with the required tolerances. This approach is considered too costly for many applications. Alternatively, the fabricated devices may go through a series of tests to xe2x80x9cweed outxe2x80x9d those receivers (perhaps a majority of devices) with unfavorable characteristics (such as, for example, bandwidth too low (that is, the 3 dB cutoff frequency is below an acceptable value), or bandwidth too large (allowing, for example, an excessive amount of noise to be received). In this case, the device xe2x80x9cyieldxe2x80x9d drops significantly, resulting in an increased cost for those devices that are found to be acceptable. Another prior art solution is to apply post-filtering beyond the receiver preamplifier so as to limit the noise from the preamplifier. Although this solution can be used successfully, it is considered to be extremely time-consuming, since it requires each receiver to be tested individually, with capacitors of the appropriate value being inserted in the circuit to provide the required filtering. The tight tolerance on the bandwidth requirement for optically amplified systems also leads to relatively low manufacturing yield as process variations move the receiver bandwidth beyond the design constraints.
A need remains in the art, therefore, for a relatively simple and straightforward arrangement for improving the control of the bandwidth of optical receivers.
The need remaining in the prior art is addressed by the present invention, which relates to a digital control arrangement for adjusting the bandwidth of an optical receiver and, more particularly, to an arrangement for modifying a parametric value (e.g., voltage, current, capacitance, etc.) applied to a preamplifier so as to tune the output bandwidth of the receiver.
In accordance with one embodiment of the present invention, a voltage regulator is used to apply the input supply voltage to the preamplifier of an optical receiver, where the preamplifier is known to display a bandwidth that is a function of the input voltage. The voltage regulator is adjustable by means of an included potentiometer. In operation, the bandwidth of the receiver is first determined. If the bandwidth is too low for the particular application, the potentiometer is adjusted to vary the resistance sensed by the voltage regulator, thus increasing (or decreasing, depending upon the polarity) the input voltage applied to the preamplifier. The change in input voltage results in increasing the bandwidth of the preamplifier, thus moving the 3 db cutoff of the receiver xe2x80x9cupwardxe2x80x9d in frequency. Conversely, if the measured bandwidth of an exemplary receiver is too xe2x80x9chighxe2x80x9d (that is, the 3 dB cutoff frequency is beyond an upper bound limit, allowing for noise signals to pass through the receiver), the potentiometer is adjusted such that the voltage applied to the preamplifier by the voltage regulator is decreased (or increased, depending upon the polarity). The change in input voltage results in decreasing the bandwidth, moving the receiver""s bandwidth xe2x80x9cdownwardxe2x80x9d into the xe2x80x9cacceptablexe2x80x9d range.
It is a feature of this arrangement of the present invention that the voltage applied as the preamplifier input voltage is regulated via the voltage regulator. In this arrangement, therefore, variations in xe2x80x9cnominalxe2x80x9d supply voltages from device to device will not affect the performance of the receiver.
In a preferred embodiment of the present invention, a digital potentiometer is used and controlled by a testing system such that the resistance is either incremented or decremented (as the case may be) in predetermined step sizes until a desired bandwidth is obtained.
In accordance with the present invention, any type of parametric control can be used to digitally change the preamplifier bandwidth. For example, a current signal, resistance, capacitance, electric and/or magnetic field, etc., can all be incremented/decremented in a predetermined manner to provide for an adjustment of the bandwidth of the preamplifier.